Water-Sediment Regulation Scheme Research Articles

Deltaic marsh accretion under episodic sediment supply controlled by river regulations and storms: Implications for coastal wetlands restoration in the Yellow River Delta

Sediment supply has long been recognized as a critical factor affecting salt marsh evolutions. Subject to fluctuating river discharges and coastal hydrodynamics, sediment supply to deltaic marshes are episodic rather than continuous. To evaluate the impacts of episodic sediment delivery under river regulations and storm events on deltaic marsh accretion, a 5-month (April 18th to September 4th, 2021) field campaign was conducted at a Spartina alterniflora marsh of the Yellow River Delta, China. Nine storm events and the Water-Sediment Regulation Scheme (WSRS) coinciding with a freshwater diversion were captured. It was found that the two phases of the WSRS differed greatly in their contribution to marsh accretion. During the Water-Regulation Phase (WRP), marsh accretion rates showed little difference with pre-WRP periods. However, the subsequent Sediment-Regulation Phase (SRP) contributed to ∼15 % of annual accretion within only 4 days, suggesting the SRP was more effective in nourishing deltaic marshes with sediment. Further analyses indicated that the higher accretion during SRP was attributed to both higher sediment concentrations and more favorable sediment pathways, influenced by delta-scale river-tide interactions. Observations also showed that storm events contributed to half of the marsh accretion during the non-WSRS period when the fluvial sediment supply was very low. During storms, wave-induced intertidal resuspension acted as the major sediment sources. Under favorable conditions (onshore wind, high water levels, high waves), tidal-averaged sediment delivery to marshes underwent as much as fifty-fold increase compared with the calm period. The observed marsh dynamics suggested that marsh sedimentations depended on not only the seaward sediment concentrations, but also the sediment transport pathways. For deltaic marsh restoration practices, we proposed that marsh-orientated schemes which align river regulations with favorable meteorological and hydrodynamic conditions, could potentially make better use of fluvial sediment for deltaic marsh nourishment.

Impacts of Water-Sediment Regulation Scheme on Chromophoric Dissolved Organic Matter in the Lower Yellow River

Impacts of Water-Sediment Regulation Scheme on Chromophoric Dissolved Organic Matter in the Lower Yellow River

How the Water-Sediment Regulation Scheme in the Yellow River affected the estuary ecosystem in the last 10 years?

The Yellow River, renowned as the most sediment-laden river globally, grapples with sediment deposition issues compromising reservoir functionality and elevating downstream riverbeds, posing threats to human life and property safety. In response, the Water-Sediment Regulation Scheme (WSRS) has been innovatively implemented to address these challenges. While effectively mitigating sediment deposition, WSRS has concurrently disrupted the equilibrium of the estuarine ecosystem. This paper addresses the understudied but crucial topic of the interannual impact of WSRS on the estuarine ecosystem. Drawing upon physical, chemical, and biological data gathered through field surveys conducted before, during, and after WSRS from 2011 to 2022, the analysis delves into the interannual changes in the estuarine environment, fish eggs and larvae abundance, and species diversity under the influence of WSRS. The findings reveal an interannual decreasing trend in terrestrial material input due to WSRS, juxtaposed with an interannual increasing trend in fish eggs and larvae around the estuary, as well as the species diversity index. Notably, these trends became more pronounced post-2014. Compared to pre-2014, nutrient concentrations experienced a ~20 % decrease, chlorophyll-a concentration increased by 44 %, fish eggs proliferated approximately 1 time, and the species diversity index transitioned from a declining trend to an ascending trajectory. After 12 years of continuous WSRS implementation, the impact on the estuarine ecosystem has demonstrably diminished. This research aims to furnish reference experience and scientific basis for water and sediment regulation in major rivers around the world in terms of estuarine ecology.

Operational Mode for Water–Sediment Regulation in Plain-Type Sand-Laden Reservoirs: A Case Study of the Haibowan Reservoir

Excessive sedimentation in sand-laden rivers significantly hinders the normal operation and overall effectiveness of reservoirs. This is observed particularly in plain-type sand-laden reservoirs where weak hydraulic conditions in the reservoir area contribute to sediment deposition. Water–sediment regulation is essential in reducing sedimentation and prolonging the lifespan of such reservoirs. Taking the plain-type reservoir of the primary stream of the Yellow River—Haibowan Reservoir as an example, based on a two-dimensional shallow water equation, a two-dimensional mathematical model of water and sediment in the reservoir area is established, the corresponding boundary conditions are improved, and the reliability of the model is verified. Under the premise of ensuring the flood control safety of the reservoir dam, considering the impact on the power generation of the reservoir and the downstream river, and maintaining the long-term large effective storage capacity of the reservoir as the basic principle, the water–sediment regulation scheme is proposed. A two-dimensional kinetic model of sediment transport and representative water–sediment series are employed to simulate the erosion and sedimentation processes in the reservoir under different schemes. The optimisation and comparison of multiple schemes reveal that the sediment-flushing scheme, which lowers the water level to 1072 m above sea level when the inflow discharge is 1500–2760 m3/s and inflow sediment concentration is 5–10 kg/m3, can yield an effective reservoir capacity of 97 million m3 and an average annual hydropower generation of 389.5 million kWh after 15 years of operation. This scheme outperforms the existing sediment-flushing scheme and is recommended as an optimised approach for water–sediment regulation in reservoirs. The results of this study provide technical support for the application of water–sediment regulation in the Haibowan Reservoir and may be useful in the exploration and management of water–sediment regulation for plain-type reservoirs on sand-laden rivers.

Dynamic evolution characteristics of the Yellow River Delta in response to estuary diversion and a water–sediment regulation scheme

River deltas have received increasing attention over the past 50 years due to their socioeconomic importance and unique ecology. In this study, long-term hydrological data and remotely sensed satellite images were used to analyze the influence of estuary diversion and a water–sediment regulation scheme (WSRS) on the Yellow River Delta. The results show that the evolution of the delta’s area since the 1980s can be divided into three stages: a rapid accretion stage (1985–1996), a retreat stage (1997–2001) and a recovery stage (2002–2020). The efficiency of sediment feeding the delta through the Qing 8 course during 1997–2020 (24.05 km2 (109 t)−1) was slightly lower than that through the Qingshuigou course during 1985–1995 (25.63 km2 (109 t)−1). For the old Qingshuigou lobe and the new Qing 8 lobe, critical sediment demands are estimated to be 303.9 × 106 t and 44.1 × 106 t per year, respectively, to maintain area equilibrium. The distribution of sediment sources for the delta’s growth have changed due to the implementation of the WSRS: the fraction of annual average net sediment flux from the river’s middle reach decreased from 87.1 % to 43.5 %, while that from the lower reach increased from zero to 19.7 %.

Detecting the magical yellow-blue demarcation off the Yellow River Estuary from the space

River deltas rank among the most ecologically and socio-economically valuable environments on the planet. The Yellow River Delta in China is not only home to young wetland ecosystem, but also to world-renowned tourism resources. One of these was the unique yellow-blue demarcation line (YBDL), formed in the Yellow River Estuary (YRE) as a result of the meeting of terrestrial sediment-laden river water and clear sea water, which had developed into a famous attraction and generated significant tourism revenues. Moreover, its distribution was of vital ecological and geomorphological importance due to association with the dynamics of estuarine turbidity plume. However, little is yet known about the dynamics and influencing factors of YBDL. In this study, we proposed an integrated method for extracting YBDL from satellite imagery and investigated the spatiotemporal distributions of YBDL during the flood season (June-October) from 2016 to 2022 using multi-source satellite imageries. The average relative error of our algorithm reached 10.53%. The results indicated that the YBDL started to frequently occur after 2018, mainly distributed within 10 km off the YRE, with two types of short straight and long curved shapes. July was the period with the highest frequency and most widespread distribution of YBDL. The riverine water and sediment discharge controlled 45.9% of the variations in YBDL length, and the extreme flooding, as well as the implementation of Water-Sediment Regulation Scheme, significantly increased the distribution extent of the YBDL. The morphology of river distributary mouths dominated the changes in the YBDL distribution centres, and the north distributary mouth experienced dramatic evolution in the last three years, with an increased YBDL distribution. Tidal currents could promote the development of YBDL at the front of the river mouth plumes while inhibiting the YBDL on the other side. This study is the first systematic investigation of the spatiotemporal evolutions and driving mechanisms of YBDL in the YRE, which could provide a reference for tourism planning and YRE National Park construction.

Potential ecological risks of heavy metals and Cd accumulation characteristics of Suaeda salsa under different Cd input and water logging conditions in the Yellow River estuary, China.

To improve the remediation of heavy metal pollution by typical wetland vegetation and maintain the health of wetland ecosystems under the water-sediment regulation scheme (WSRS) application, we evaluated the potential ecological risk of heavy metals in surface sediment in the Yellow River estuary affected by the WSRS. The ranges of Cr, Cu, Zn, Cd, and Pb content in surface sediment were 52.44-100.80 mg·kg-1 dry weight (DW), 16.38-21.19 mg·kg-1 DW, 64.77-255.50 mg·kg-1 DW, 0.12-0.24 mg·kg-1 DW, and 5.40-8.63 mg·kg-1 DW, respectively, and potential ecological risk coefficients showed that Cd was associated with moderate potential risk. We further examined effects of Cd in a greenhouse experiment to explore the influence of short-term Cd input and water logging condition changes induced by WSRS on the Cd absorption characteristics of Suaeda salsa (L.) Pall in the Yellow River estuary. The results showed that total biomass decreased but Cd content in tissue of S. salsa increased with increasing Cd input and the accumulation factor reached maximum values at 100 μg·L-1 of Cd, indicating that S. salsa efficiently accumulated Cd. Water logging depth significantly affected S. salsa growth and Cd absorption with deeper water logging being detrimental to growth. The interaction effect of Cd input and water logging depth on Cd content and accumulation factor was significant. These results suggest that WSRS caused short-term heavy metal input and changes in water conditions affect wetland vegetation growth and heavy metal absorption in the downstream estuary.

Changes of active particulate uranium under the Water-Sediment Regulation Scheme in the lower Yellow River: Potential impact to the uranium flux into the global ocean

With the annual Water-Sediment Regulation Scheme (WSRS) transporting large amounts of suspended particulate matter (SPM) to the sea within several days, the behavior of uranium in the Yellow River during the WSRS is crucial to better understand the uranium flux. In this study, the active forms (the exchangeable, carbonate bounded, Fe/Mn oxides bounded, organic matter bounded) and the residual form of particulate uranium were extracted by the sequential extraction method and their uranium contents were measured respectively. Results show that the content of total particulate uranium was 1.43–2.56 μg/g and the active forms accounts for 11–32 %. Particle size and redox environment are the two main factors controlling the active particulate uranium. The flux of active particulate uranium at Lijin was 4.7 tons during 2014 WSRS, which was about 50 % of the dissolved uranium flux for the same period. Thus, the terrestrial uranium flux is significantly modified by artificial regulation.

Dynamic transport of particulate organic carbon in the Yellow River during dam-orientated Water-Sediment Regulation

The Yellow River has experienced prolific environmental changes from dam regulations that have significantly altered the downstream hydrologic regime, and transport of sediment and POC within the river-coast continuum. Here, we investigated the contrasting changes in sources and transport of POC from the lower Yellow River to the river mouth, at two different stages (e.g., Water vs. Sediment regulation) of dam operation - during the period of Water-Sediment Regulation Scheme (WSRS). Riverbed erosion downstream the Xiaolangdi Dam contributed the POC with more enriched δ13C stable isotopic values, along with those more isotopically depleted POC from riparian floodplains and the reservoir at the Water-regulation stage, when clear water was released from the reservoir. At the Sediment-regulation stage, muddy floodwaters were released, while soil-derived POC and fine-grained sediment released from the reservoir became dominant with evidently increasing POC content. The dispersal and distribution of river-laden POC at the river mouth were primarily modulated by the plume dynamics, as remotely impacted by dam regulation, in combination with the dynamic shear front. As a result, the POC distribution was confined within 10 km off the river mouth due to the rapid deposition of coarse-grained sediments. In contrast, the fine-grained sediment and more isotopically depleted POC, from modern sources (e.g., C3 vascular plant), were delivered to the far-field plume region. POC transport along the Yellow River-Estuary continuum exhibited complex behaviors, coupled with natural and dam-regulated drivers, on a variety of spatial-temporal scales. These findings provide important insights into dynamic linkages on how coastal carbon cycling can be significantly altered, remotely from the upstream dam regulations, in a large global river system.

Community structure characteristics of ichthyoplankton during the water-sediment regulation scheme in the Yellow River in 2020 and 2021

The Water-Sediment Regulation Scheme (WSRS) will deliver large amounts of water and sand to the Yellow River basin within a short period of time. This will significantly change the physicochemical environment of the Yellow River estuary and the surrounding marine ecosystem. Its effects on the spatial and temporal distribution patterns of ichthyoplankton are still unknown. In this study, six surface horizontal trawl surveys of ichthyoplankton were conducted during the WSRS in 2020 and 2021 using plankton nets. The results were as follows: (1) the estuarine sedentary fish Cynoglossus joyeri was the main species controlling the succession pattern of summer ichthyoplankton communities in the Yellow River estuary. (2) The WSRS influenced the ichthyoplankton community structure by changing the runoff, salinity, and suspension environment in the estuary. (3) The northern and southeastern parts of the estuary near Laizhou Bay were the main aggregation areas of the ichthyoplankton community.

Modeling the effect of artificial flow and sediment flux on the environment and plankton of an estuary

Estuarine environments are influenced by both river flows and oceanic tidal movement of water, sediment, and nutrients, often forming ecosystems that are rich in resources and biodiversity. The Yellow River once carried the world's largest sediment load, but artificial structures have transformed its hydrodynamic processes. An annual Water–Sediment Regulation Scheme (WSRS) was introduced to flush accumulated sediment from the Xiaolangdi Reservoir, which provides flood control and water storage. However, the effect of the WSRS process on the runoff and sediment conditions, nutrients, and ecological environment of the Yellow River Estuary and adjacent ocean are not well understood. In the current study, a coupled hydrodynamic–nutrient–plankton ecosystem dynamics model (FVCOM–FABM–NPZD) was constructed to simulate changes in the ecological parameters and in the vertical response in the Yellow River Estuary before, during, and after the WSRS, This model also was used to quantify the effects of changes in the incoming material flux on the hydrodynamic and ecological environment of the estuary, using the Yellow River Xiaolangdi Reservoir water transfer and sand transfer as an example. The study found that the WSRS changed the spatial and temporal distribution of temperature, salinity, sediment, inorganic nitrogen, and phytoplankton in the Yellow River Estuary and adjacent waters. It also had a significant effect on the temperature, salinity, and ecology of the adjacent near-shore marine environment. The spatial and temporal responses of zooplankton and phytoplankton to the WSRS differed. Zooplankton showed a lag in response to the WSRS and were mainly influenced by temperature and phytoplankton. The phytoplankton concentration was positively influenced by inorganic nitrogen and negatively influenced by salinity and sediment. In the current study, the effects of changes in the flow and sediment flux from the WSRS on environmental factors and zooplankton in the Yellow River Estuary were simulated, providing a theoretical basis for scheduling the release of water and sediment in the Yellow River basin and providing a reference for water and sediment regulation in other reservoirs upstream of the estuary.

Numerical study of the water-sediment regulation scheme (WSRS) impact on suspended sediment transport in the Yellow River Estuary

IntroductionThe transport of suspended sediment plays an important role in regulating erosion-accretion in estuaries. The Yellow River, the second longest river in China, which has a large amount of sediment and contributes sediment to the Yellow River Estuary (YRE) every year. The water and sediment discharge patterns in the lower reaches of the Yellow River have experienced significant changes over recent decades. In particular, the water-sediment regulation scheme (WSRS) of the Xiaolangdi Reservoir transports large amounts of water and sediment to the YRE within a very short time, causing dramatic changes in the spatiotemporal estuarine sediment dynamics.MethodsIn this study, we presented a coupled numerical model based on FVCOM-SWAVE-SED that considered the highly dynamic sediment variations in the YRE. The sediment distribution and erosion-accretion patterns in the YRE during the WSRS in 2013 were analyzed by the high-resolution model.ResultsThe sediment entering the YRE spread with freshwater, forming a high sediment concentration zone near the river mouth, where most of the sediments were deposited. The sediment dispersal distance was limited by the tidal shear frontal (TSF), and the southeast outspread length of the sediment was slightly larger than the northwest spread. Outside of the YRE, the areas with high flow currents exhibited high erosion. We examined the effects of the main external driving forces (such as waves, tides, and runoff) on the sediment distribution and showed that runoff, tides, and waves were the key factors affecting the sediment distribution of the YRE.ResultsThis study shows that WSRS effectively changes the sediment distribution and erosion condition in the YRE and provides a data for researches on changes in estuarine ecosystems.

Porewater dissolved inorganic carbon released due to artificial sediment scouring in the Yellow River

The water-sediment regulation scheme (WSRS) in the Yellow River has been adjusted annually to maintain the optimal capacity of the Xiaolangdi Reservoir via the scouring of deposits downstream before the rainy season. The high-intensity sediment scouring in a short time has given rise to elevated dissolved inorganic carbon (DIC) exportation; however, the source of the DIC is still unclear. Here, we hypothesized that sediment scouring released porewater DIC into the river water, increasing the riverine DIC concentration and enhancing CO2 outgassing during the adjustment of the WSRS. To test this hypothesis, we used the isotopic composition of the DIC (δ13CDIC) to constrain the sources and transformations of the DIC. River water samples were collected below the Xiaolangdi Reservoir dam before, during, and after the adjustment of the WSRS. In these three stages, the pH of the river water decreased from 8.2 to 8.0 and then increased to 8.4; and the DIC concentrations increased from 185.30 to 216.37 mg L−1, and then decreased to 199.93 mg L−1. The δ13CDIC values decreased from −7.01 to −11.53‰ and then increased to −6.82‰; and the partial pressure of the CO2 increased from 5.21 × 10−3 to 35.30 × 10−3 atm, and then decreased to 3.40 × 10−3 atm. The increase in the DIC concentrations with negative δ13CDIC values during the adjustment of the WSRS resulted from the release of porewater DIC due to the sediment scouring, which was derived from the anaerobic oxidation of organic matter in the permeable sediment environment. More broadly, the sedimentary organic carbon-derived DIC released into the ocean would counteract CO2 drawdown via the burial of organic carbon in the geological timescales.

Phenological heterogeneities of invasive Spartina alterniflora salt marshes revealed by high-spatial-resolution satellite imagery

The rapid expansion of invasive Spartina alterniflora in China’s coastal wetlands has seriously threatened regional ecosystem health. Characterizing phenology of S. alterniflora salt marshes supports the studies on plant invasion mechanism and carbon fluxes in coastal wetlands. To date, little is known about the capacity of high-spatial-resolution imagery in revealing local scale heterogeneities and temporal variations in the phenology; in addition, the driving factors of such heterogeneities are not well understood. This study took Yellow River Estuary (YRE) as the study site where S. alterniflora area has expanded since 2008. We retrieved start of season (SOS), end of season (EOS), length of growing season (LOS), and maximum Normalized Difference Vegetation Index (NDVImax) from time series Sentinel-2 imagery (10-meter resolution) during 2017 ∼ 2019 with assist of a rule-based data noise removal approach. Results showed that phenological metrics differ with invasion years. The differences in the median NDVImax reached 0.13, in the median SOS reached 26 days, and in the median LOS reached 51 days. S. alterniflora with 5-year invasion had the highest NDVImax, and younger S. alterniflora marshes had earlier SOS and longer LOS. Both SOS and EOS showed negative relationships with NDVImax, indicating the marshes with greater green vegetation cover had earlier SOS and EOS. More spatial details can be revealed from higher-resolution PlanetScope imagery (3-meter resolution), showing the influence of tidal creeks on phenology. The spatial heterogeneity in phenology was associated with environmental factors such as tidal inundation, elevation, micro-climate, nutrients, and canopy structure. S. alterniflora showed markedly higher NDVImax and earlier EOS in 2018 compared to 2017 and 2019, associated with larger river water discharge and sediment load brought by Water Sediment Regulation Scheme, greater precipitation in the wettest season and lower preseason temperature in 2018. Large phenological variations at both spatial and temporal scales suggest that S. alterniflora marshes are sensitive to the local environment and climate change. This study has the potential to provide references for the management of invasive plants threatening coastal wetlands in the world.

Hydro-geomorphological regime of the lower Yellow river and delta in response to the water–sediment regulation scheme: Process, mechanism and implication

The water and sediment regulation schemes (WSRS) orientated by dams have profoundly adjusted the hydrological process and geomorphological evolution of the lower Yellow River and delta. Two decades since the implementation of WSRS, the observed multidisciplinary datasets allowed us to systematically evaluate the impacts of this unprecedented engineering effort on the river-coastal system. The sediment fluxes transported to the sea showed a notable rebounded and coarsened process due to the erosion of the riverbed during the WSRS periods. The average median grain size of suspended sediment increased from 17.7 μm in the 1990 s to 26 μm in 2002–2009. The natural rhythm of hydrological process is fundamentally altered by WSRS, about 50% and 30% of annual sediments and water delivered to the sea for only 20 days. The water discharge system is more temporally dynamic and complex with higher dimensionality after the operation of WSRS. The erosion pattern has been a dominant feature in all the channel segments downstream the Xiaolangdi Reservoir, with an average total erosion volume of 1.02 × 108 m3/a after the WSRS. However, the erosion efficiency of the lower river showed a decreased trend, due to the riverbed had formed a coarse armored substrate for the continued scouring since 2002. The YRD has changed from the pattern of erosion to progradation for the increase of sediment supply, and the average growth rate of subaerial delta was about 6.89 km2/a during 2003–2014. The lessons learned from the operation of WSRS may provide a valuable reference for the management of global large river systems.

Lag response of groundwater to changes in water and sediment characteristics in the lower Yellow River, China

• XLD operation decreased groundwater level along the lower Yellow River. • The highest groundwater level was delayed after XLD operation. • Inter-annual changes in groundwater were mainly determined by WL, SSC, and ISC. • Response of groundwater to changes in water–sediment lagged by 2–5 months. The process of water exchange between rivers and groundwater is critical for the circulation of material and energy; however, this process is strongly affected by dam-regulated water and sediment. The lower Yellow River seeps downward throughout its coarse and suspended (perched) riverbed and feeds the surrounding floodplain groundwater. The Xiaolangdi Dam (XLD) and water–sediment regulation scheme (WSRS) have altered the exchange processes between river and groundwater along the lower Yellow River. However, few researchers have examined the response of this river-fed groundwater system to the operation of the XLD and implementation of the WSRS. As such, in this study, we analyzed the responses of groundwater to changes in flow and sediment characteristics based on statistical analysis using historical data, extremely randomized tree models, and distributed lag non-linear models. We found that, since the XLD began operating in 2002, the groundwater level near the lower Yellow River substantially decreased, and the groundwater level in June significantly decreased (at the 0.05 level). The highest groundwater level in the pre-dam period occurred in August, whereas the highest level in the post-dam period occurred in October. Changes in the annual groundwater level and in the mid- and post-WSRS seasons along the lower Yellow River were mainly determined by the water level and incoming sediment coefficient in the river, whereas those in the pre-WSRS season were mainly determined by the water level and suspended sediment concentration. Regarding the changes in groundwater level between the mid- and post-WSRS seasons, we found that suspended sediment concentration was the dominant factor in the pre-dam period, whereas the combined effect of water and sediment was the dominant factor in post-dam period. The response of the monthly groundwater level to changes in water and sediment characteristics lagged by 2–5 months. These results are essential to understanding the interaction between river water and groundwater.

Relative contribution of multi-source water recharge to riparian wetlands along the lower Yellow River

Rivers play a vital role in both the formation and maintenance of riparian wetland hydrology. However, few studies have focused on the response of water recharge of riparian wetlands to altered hydrological processes induced by water-sediment regulation practices. To fill this gap, our study investigated the contribution of multi-source water recharge of riparian wetlands in the lower Yellow River, as well as its influence both during and before the water-sediment regulation scheme of Xiaolangdi Dam. Our study is based on hydrochemistry and isotopic methods, using a Bayesian mixing model and artificial neutral network model. The results showed that riparian wetlands were fed by mixed sources, including groundwater, canals, the Yellow River, and precipitation. However, seasonal evaporation introduced additional variation, which affected the relative contribution of these sources across seasons. Among these sources, the Yellow River served as the main water source for recharging riparian wetlands, and its contribution varied both spatially and temporally (across seasons). Specifically, proximity of riparian wetlands was the primary factor explaining spatial variation in the contribution of Yellow River, while climatic (12.38%) and hydrological variabilities (87.62%) explained seasonal variation. Among these climatic and hydrological variables, suspended sediment content was the most important factor—with a relative contribution of 36.33%. By determining the contribution of the Yellow River to the recharge of riparian wetlands, our study has provided information which is beneficial to adaptive management of river-fed riparian wetlands, especially under the implementation of water-sediment regulation practices.

Impact of a water-sediment regulation scheme on nutrient variations at the Lijin station of the Yellow River

The water-sediment regulation scheme (WSRS) imposed on dams throughout the Yellow River not only alleviates siltation in the downstream section but also alters the nutrient characteristics, which indirectly affects the enrichment of nutrients in the estuary. Nevertheless, the long-term changes in the nutrient contents and their causes in the lower Yellow River (LYR) remain unclear, and the nutrients characteristics during the years with and without WSRS have yet to be compared. Therefore, the purpose of this study was to explore the variations in the nutrient contents and limitations at the Lijin station on the LYR over the past decade, especially during the annual WSRS period, and to compare the water quality characteristics at Lijin between the years with and without WSRS. The results reveal that WSRS significantly changed the seasonal nutrient concentrations (nitrogen, phosphorus and silicon) at the Lijin station. The fluxes of these nutrients during WSRS (excluding 2016 and 2017) accounted for 11.64–40.63% of the total annual fluxes. The N concentration in the LYR was higher than that in some global rivers, while the concentrations of dissolved inorganic phosphorus (DIP) and dissolved silica (DSi) were lower than the average levels in other rivers. In addition, higher values of dissolved inorganic nitrogen (DIN), DSi and the Redfield ratio indicated that the growth of phytoplankton at the Lijin station was strongly restricted by P. However, during the 2 years without WSRS (2016 and 2017), the proportions of the nutrient fluxes in June were less than 66% of those in the WSRS period in other years. Additionally, there was a potential Si limitation in June in these 2 years. Furthermore, due to the occurrence of floods upstream of the Yellow River and the low-level operation of the Xiaolangdi Reservoir, the fluxes of nutrients during WSRS in 2018 were approximately 0.90–4.20 times those during the same period in 2009–2015 and 6.30–35.76 times those in June 2016 and June 2017. This study shows that WSRS effectively changes the nutrient balance in the LYR and provides a reference for the multi-objective collaborative optimization of WSRS to improve siltation and control flood in the LYR.

Spatial and temporal changes in nutrients associated with dam regulation of the Yellow River

Sediment regulation via manipulating dam operations is an effective solution for dealing with reservoir siltation and downstream channel incision. Large amounts of sediment and thereby nutrients can be discharged within a short period of time. To ascertain the concentration of and transformation between dissolved and particulate nutrients during such periods, we conducted field observations in the Xiaolangdi Reservoir and its downstream reach during the sediment regulation stage of the Water-Sediment Regulation Scheme (WSRS) in 2018. We found that water and sediment characteristics differed significantly pre- and post-WSRS period. Nutrient concentrations varied significantly among the dissolved, suspended, and deposited phases, while no significant differences were observed between the reservoir and downstream reach, except for dissolved total carbon, particulate ammonium nitrogen, and calcium-bound phosphorus. Total nitrogen (TN) and total phosphorus (TP) concentrations of the dissolved phase were significantly positively correlated with suspended sediment concentration (SSC) and electrical conductivity (EC) (p < 0.05). Total organic carbon (TOC) concentration of the suspended phase was significantly negatively correlated with EC and median particle size (D50) (p < 0.05). Nutrient concentration during the deposited phase was less correlated to selected hydrological and physical parameters. Nutrient transformation between the deposited phase and the suspended and dissolved phases were higher than those between suspended and dissolved phases. TP transformation was closely related to hydrological parameters (SSC and D50), while TN transformation was predominantly related to physical parameters (EC and DO). The concentration and transformation of nutrients during the transport process from the reservoir to downstream were likely driven by different factors. In conclusion, reservoir management practices like WSRS could mitigate nutrients retention in reservoir, however, leading to higher nutrients transformation downstream.

Influences of reservoir operation on terrestrial water storage changes detected by GRACE in the Yellow River basin

The implementation of water–sediment regulation scheme (WSRS) in the Yellow River can induce instantaneous water redistribution over regions through frequent reservoir operation. In this study, we attempt to detect the signal of changes in terrestrial water storage (TWS) induced by reservoir operation during WSRS based on GRACE data. Meanwhile, a modified index is proposed to further assess the contributions of human activities and climatic variability to annual changes in TWS. The results indicated that the impounding or releasing events in the XLD reservoir and the LYX reservoir during WSRS can be well detected by monthly GRACE CSR solutions with the DDK3 filter. In addition, there exists a significant correlation between annual changes in TWS and reservoir water storage changes within both the catchment area above the LYX reservoir (rs = 0.88, p < 0.01) and the catchment area above the XLD reservoir (rs = 0.86, p < 0.01). The modified index proposed in this study, which is an attribution index based on a variation analysis, showed that reservoir operation contributes significantly to changes in TWS especially for heavily reservoir-regulated regions.